Hello!,
On 2024-06-25 at 09:28:55 -0700, Reinette Chatre wrote:
Hi Maciej,
On 6/25/24 4:04 AM, Maciej Wieczor-Retman wrote:
Hello, sorry it took me so long to get back to this. I prepared the next version with your comments applied and Tony's replies taken into account.
Thank you very much for sticking with this.
I wanted to briefly discuss this before posting:
On 2024-05-30 at 16:07:29 -0700, Reinette Chatre wrote:
On 5/15/24 4:18 AM, Maciej Wieczor-Retman wrote:
return 1;
- }
- for (i = 1; i <= MAX_SNC ; i++) {
if (i * node_cpus >= cache_cpus)
return i;
- }
This is not obvious to me. From the function comments this seems to address the scenarios when CPUs from other nodes are offline. It is not clear to me how this loop addresses this. For example, let's say there are four SNC nodes associated with a cache and only the node0 CPUs are online. The above would detect this as "1", not "4", if I read this right?
I wonder if it may not be easier to just follow what the kernel does (in the new version). User space can learn the number of online and present CPUs from /sys/devices/system/cpu/online and /sys/devices/system/cpu/present respectively. A simple string compare of the contents can be used to determine if they are identical and a warning can be printed if they are not. With a warning when accurate detection cannot be done the simple check will do.
Could you please add an informational message indicating how many SNC nodes were indeed detected?
Should the information "how many SNC nodes are detected?" get printed every time (by which I mean at the end of CMT and MBM tests) or only when we get the error "SNC enabled but kernel doesn't support it" happens? Of course in the first case if there is only 1 node detected nothing would be printed to avoid noise.
I agree that it is not needed to print something about SNC if it is disabled. hmmm ... so SNC impacts every test but it is only detected by default during CAT and CMT test, with MBA and MBM "detection" only triggered if the test fails?
Yes, snc_ways() ran before starting CAT and CMT to adjust cache size variable. And then after CAT,CMT,MBM and MBA if the return value indicated failure.
What if the "SNC detection" is moved to be within run_single_test() but instead of repeating the detection from scratch every time it rather works like get_vendor() where the full detection is only done on first attempt? run_single_test() can detect if SNC is enabled and (if number of SNC nodes > 1) print an informational message that is inherited by all tests. Any test that needs to know the number of SNC nodes can continue to use the same function used for detection (that only does actual detection once).
What do you think?
I think running the detection once at the start and then reusing the results is a good idea. You're proposing adding a value (global or passed through all the tests) that would get initialized on the first run_single_test()?
And then the SNC status (if enabled) + a warning if the detection could be wrong (because of the online/present cpus ratio) would happen before the test runs?
On the warning placement I think it should be moved out of being printed only on failure. I did some experiments using "chcpu" to enable/disable cores and then run selftests. They didn't have any problems succeeding even though SNC detection detected different mode every time (I added a printf() around the line that cache size is modified to show what SNC mode is detected). While I understand these tests shouldn't fail since they just use a different portion of the cache I think the user should be informed it's not really NUMA aware if the detection was wrong:
(this was a 2 socket machine with SNC-2 and 55296K L3 cache size)
This is without any changes: [root]# ./resctrl_tests -t CMT # dmesg: [ 11.464842] resctrl: Sub-NUMA Cluster mode detected with 2 nodes per L3 cache ... SNC NODES DETECTED : 2 # Cache size :28311552 ... # Average LLC val: 12413952 # Cache span (bytes): 11796480 ok 1 CMT: test
This is with all cores on node 1 disabled: [root]# ./resctrl_tests -t CMT # dmesg: [ 11.464842] resctrl: Sub-NUMA Cluster mode detected with 2 nodes per L3 cache ... SNC NODES DETECTED : 1 # Cache size :56623104 ... # Average LLC val: 22606848 # Cache span (bytes): 23592960 ok 1 CMT: test
And this with one core on node 0 disabled: [root]# ./resctrl_tests -t CMT # dmesg: [ 11.464842] resctrl: Sub-NUMA Cluster mode detected with 2 nodes per L3 cache ... SNC NODES DETECTED : 3 # Cache size :18874368 ... # Average LLC val: 7382016 # Cache span (bytes): 7864320 ok 1 CMT: test
CAT also succeeds although it reports bigger or smaller cache miss rates than normally:
SNC NODES DETECTED : 1 <-- all cpus on node 1 offline # Percent diff=12.7 # Percent diff=10.2 # Percent diff=7.8 # Percent diff=6.7 ok 1 L3_CAT: test
SNC NODES DETECTED : 2 <-- real # Percent diff=49.6 # Percent diff=37.8 # Percent diff=22.4 # Percent diff=16.0 ok 1 L3_CAT: test
SNC NODES DETECTED : 3 <-- one cpu on node 0 offline # Percent diff=76.6 # Percent diff=53.3 # Percent diff=35.1 # Percent diff=28.9 ok 1 L3_CAT: test
Reinette